The present invention relates to a loss-in weight feeder (LWF) that utilizes a vibratory tray to convey material.
Vibratory feeders exhibit many advantages over other types of material feeding systems such as belt feeders or motor driven auger feeders. For example, non-vibratory feeders, while exhibiting excellent long-term control over material feeding, often have non-uniform material feeding over the short term. Non-vibratory feeders are often unacceptable for applications, such as ingredient mixing, that require that specified material ratios be maintained at all times.
Vibratory feeders are also mechanically simpler than these other types of feeders because they operate without bearings, motor brushes, seals and the like. This results in a higher reliability and lower cost material feeder compared with other types of material feeders, and since vibratory feeders typically have no sparking electrical contacts, they are readily adaptable to highly explosive hazardous environments.
In addition, since the only component of the vibratory material feeder that is in direct contact with the material being fed is a feed tray, or the like, cleaning is greatly simplified. Vibratory feeders also exhibit excellent temperature stability, and are capable of operating with high efficiency; for example, feeding 25 tons of material per hour with only 60 watts electrical input.
Howevers commercially available vibratory feeders are not without disadvantage. Material feed rate is not easily controllable because of the non-linear relationship existing between the actual feedrate and the vibratory drive voltage. For example, doubling the drive voltage may result in an actual feedrate increase of ten times.
Another disadvantage of the vibratory feeder is that the feedrate is not zero based. That is a power greater than zero must be applied before the material will begin feeding.
In addition, when materials having different physical properties are used, or when mechanical changes are made to the vibratory feeder, such as changing the feeding tray or changing the springs, the resulting performance may be radically changeds and the mass flow rate may vary from the desired setpoint rate. Nevertheless, users may continue operating the feeding system while changing materials to avoid the need to shut down other equipment receiving material from the feeding system, such as an extruder, or molding equipment.
Another disadvantage of vibratory feeders is that with some material, a build up occurs on the feeding tray that alters feeding performance of the machine over time.
To users of the vibratory feeder, the above disadvantages are cause for concern, because typically the vibratory feeder is dosing material at some desired feedrate (setpoint) into an extruder. The extruder is not tolerant to over feeding and built-in safety circuits in the extruder will shut down the entire system if significant overfeeding occurs.